The digitization of analog AC voltages using A/D converters is generally known. A special A/D converter which is often used is the so-called delta-sigma converter which makes it possible to rapidly digitize an analog AC signal with a high resolution of 24 bits, for example. The data stream (of the 24-bit data words) generated by a delta-sigma converter is generated in this case at a firmly predefined operating clock rate applied from the outside. The operating clock rate is greater than the highest frequency to be resolved in the input signal by at least the oversampling factor; if said frequency is 100 kHz, an operating clock rate of 2.56 MHz is expedient for an oversampling factor of 256.
If an FFT (Fast Fourier Transformation) is intended to be calculated for a digitized AC voltage, a fixed number of sampling points per period is required for each period of the AC voltage, preferably 2n sampling points per period, where n is a positive integer, the FFT being calculated over a multiplicity of periods which form the so-called measuring period over which the fundamental frequency of the AC signal is substantially constant.
If the fundamental frequency of the AC signal changes, the fixed operating clock rate of the output current of the delta-sigma converter must be converted into a data stream having the desired fixed number of sampling points per period, for example by means of interpolation. This is computation-intensive and relatively complicated.
Alternatively, however, use may also be made of successive approximation A/D converters which are relatively expensive and whose accuracy is limited.